Immune complexes (IC) play an important role in the pathogenesis of certain human diseases, such as serum sickness, glomerulonephitides, and lupus erythematosus, and are also associated with rheumatoid arthritis and a variety of vasculitides. Present tests for the detection of IC rely on physiochemical methods or biologically based assays which separate IC from free antibody (Ab). The various tests used do not correlate well with each other, have problems in their sensitivity and/or specificity, and do not distinguish IC of different immunoglobulin isotypes or subclasses. There is a real need for a more specific, sensitive, and easy IC assay for use in diagnosis, determining prognosis, and monitoring therapeutic intervention. We propose to develop monoclonal antibodies (M.Ab) specific for IC which do not react with either free antigen (Ag) or Ab. These M.Ab may enable IC of defined isotypes to be detected in a homogeneous assay without separation of bound from unbound Ab. To demonstrate the feasibility of this approach, we have selected a murine system with well-defined antigens (haptens) and well-defined antigen or hapten-specific antibodies of different isotypes. Both monovalent hapten (dinitrophenol, DNP) and polyvalent DNA antigens will be used. First, we will develop M.Ab which react with hapten-carrier-antihapten or DNA-anti DNA immune complexes, but not with Ag or Ab alone. Second, we will determine if any of the IC-specific M.Ab are specific for certain isotypes. Third, we will demonstrate that the epitopes recognized by these M.Ab are due to conformational changes in the bound Ab and not simply due to aggregation or dimer formation. Finally, we will begin to test these M.Ab in assays for IC in sera from mice with autoimmune disease.